PlainISA · 2026/27 · HMRC rules
UK ISA calculator & decision engine
Model the £20,000 adult allowance across all five HMRC wrappers. Project Cash, Stocks & Shares, and Lifetime paths with sourced rates: every assumption traces to HMRC or the underlying index series.
Quick allowance projection
Enter contribution, horizon, and wrapper. Opens the full calculator with your inputs for 2026/27.
PlainISA models the £20,000 adult ISA allowance for 2026/27 across 5 HMRC wrappers. Lifetime subscriptions cap at £4,000 with a 25% HMRC top-up (max £1000/year); Junior ISAs use a separate £9,000 per-child allowance. Cash ~4.5% with FSCS to £85,000; FTSE All-Share real 30-year near 5%.
- 5 HMRC wrappers covered
- LISA 25% bonus capped at £1000/year
- 2026/27 HMRC rules + live registry returns
The national picture
One adult pot of £20,000 covers Cash, Stocks & Shares, Lifetime, and Innovative-Finance ISAs in 2026/27. Lifetime subscriptions cap at £4,000 (25% HMRC top-up); Junior ISAs sit on a separate £9,000 per-child allowance. Cash averages ~4.5% with FSCS cover to £85,000; FTSE All-Share real 30-year returns sit near 5%.
- £20,000
- Adult annual cap
- 5
- HMRC wrappers covered
- 4.5%
- Cash ISA ~1y avg
- 5%
- FTSE real 30y CAGR
According to HMRC 2026/27 tax-year rules, allowance and bonus figures above are the published thresholds. Return figures are live series from the portal registry (cash_isa_avg · FTSE All-Share TR inflation-adjusted), not broker quotes.
How the £20,000 adult maximum is built in the illustration
HMRC caps Lifetime subscriptions at £4,000 inside the £20,000 adult pot. The charts below use that statutory maximum split: £4,000 Lifetime + £16,000 Stocks & Shares. You can allocate the pot differently; this is one additive reading of the rules, not a recommendation.
Registry rates versus a maximum-allowance illustration
Left: the live cash ~1y average against the FTSE All-Share real 30-year series from the portal registry. Right: an illustration of the maximum £20,000 adult split (£4,000 LISA + £16,000 S&S) compounded at that FTSE real rate, not a personal forecast.
Cash ~1y vs FTSE real 30y
Same series the calculator and decision engine read. The dial is cash; the tick is the FTSE real 30-year CAGR (0.5 pp apart in the current vintage).
Projected balance, 25-year LISA + S&S split at 5% real
Maximum £20,000 allocated £4,000 LISA + £16,000 S&S, compounded at the registry FTSE All-Share real 30-year rate (5%). Illustration only.
Does the cash-vs-equity gap hold at every horizon?
The only negative real return in this series is Cash ISA · 5y at -0.5% per year, Cash ISA money actually lost purchasing power over that window. The best real return, S&S ISA (FTSE) · 30y at 5% per year, came from the equity wrapper.
Real annualised return by holding period: Cash ISA · 30y, 30 years, 0.4% real annualised return. Cash ISA · 20y, 20 years, 0.3% real annualised return. Cash ISA · 10y, 10 years, 0.1% real annualised return. Cash ISA · 5y, 5 years, -0.5% real annualised return. S&S ISA (FTSE) · 30y, 30 years, 5% real annualised return. S&S ISA (FTSE) · 20y, 20 years, 4.6% real annualised return. S&S ISA (FTSE) · 10y, 10 years, 4.2% real annualised return. S&S ISA (FTSE) · 5y, 5 years, 4.2% real annualised return.
What these two charts are saying
Rates and projected balances use different denominators, do not read the dial as a year-25 outcome.
- Cash ~4.5% (registry ~1y average) sits 0.5 pp below the FTSE All-Share real 30-year CAGR (5%). Both figures are published series, not broker quotes. Methodology
- The bar chart compounds the £20,000 adult maximum (£4,000 LISA with 25% HMRC top-up + £16,000 S&S) at that FTSE real rate for 25 years, about £1,052,383 in today's money. Change the inputs in the calculator. Open calculator
- POSTURE-B registry framing: PlainISA does not recommend which wrapper to open. Use the decision engine to see the rule constraints for your own answers. Run decision engine
Past index performance is not a guide to future returns. Allowance and bonus figures are HMRC tax-year rules and change when HMRC publishes a new year.
How the wrappers trade off
Cash versus Stocks & Shares is mostly a time-horizon call. Cash suits money you may need within five years; equity wrappers have paid higher real returns over multi-decade windows with drawdowns of 20–30% along the way. The Lifetime ISA is a 25% pay-rise on the first £4,000 of long-term saving if you can live with the lock-in until 60 or a first-home purchase below £450,000.
The bar chart above projects the maximum £20,000 split (£4,000 LISA + £16,000 S&S) at the registry FTSE real rate of 5%. After 25 years that path is about £1,052,383 in today's money. Use the calculator with your own numbers.
Maximum £20,000 split at 5% real return, 25-year projection
| Year | LISA balance (£4K + 25% bonus) | S&S ISA balance (£16K/year) | Combined |
|---|---|---|---|
| 1 | £5,250 | £16,800 | £22,050 |
| 5 | £29,010 | £92,831 | £121,840 |
| 10 | £66,034 | £211,309 | £277,343 |
| 15 | £113,287 | £362,520 | £475,807 |
| 20 | £173,596 | £555,508 | £729,104 |
| 25 | £250,567 | £801,815 | £1,052,383 |
Real (inflation-adjusted) illustration at the portal registry FTSE All-Share Total Return 5% 30-year CAGR (deflated). LISA strand assumes maximum £4,000 contribution plus the £1,000 government bonus credited annually. Past performance is not a guide to future returns; equity returns vary substantially over shorter windows. This is not personal investment advice.
All five ISA types at a glance
Who can open an ISA
UK residents aged 16 or over may open a Cash ISA; all other HMRC wrappers require age 18+. You must be UK tax-resident when you subscribe. Non-residents may keep existing ISAs but cannot add new subscriptions until residency returns. Junior ISAs are opened by a parent or guardian for a child under 18.
Age and UK residency
Cash ISA eligibility starts at 16; Stocks & Shares, Lifetime, and Innovative Finance ISAs start at 18. Lifetime ISA subscriptions stop at age 50 (the account may stay open). HMRC treats residency by the tax-year rules on gov.uk, not by citizenship alone.
One Lifetime ISA per tax year
Since 6 April 2024 you may subscribe to multiple Cash or S&S ISAs in one tax year, but you may still subscribe to only one Lifetime ISA per year and hold only one LISA account at a time. Switching LISA providers requires a transfer, not a second subscription.
Putting money into an ISA
Subscriptions are new money paid in during the tax year (6 April–5 April). Transfers from another provider do not count against the £20,000 annual pot. HMRC tracks subscriptions across all your providers; exceeding the cap voids tax-free status on the excess.
How subscriptions count against the allowance
Flexible Cash ISAs let you withdraw and replace subscriptions within the same tax year without using extra allowance. Non-flexible wrappers treat withdrawals as permanent unless you transfer out. Lifetime ISA subscriptions are capped at £4,000 inside the adult pot.
Example: splitting £20,000 across wrappers
A saver might subscribe £4,000 to a Lifetime ISA (receiving up to £1000 HMRC bonus), £10,000 to a Cash ISA, and £6,000 to a Stocks & Shares ISA in the same tax year, total £20,000. Junior ISA subscriptions use the separate £9,000 per-child allowance and do not reduce the adult pot.
What you can include in your ISAs
Cash ISAs hold deposit accounts; Stocks & Shares ISAs hold qualifying shares, funds, bonds, and investment trusts; Lifetime ISAs may be cash or investments subject to the same qualifying-asset rules; Innovative Finance ISAs hold peer-to-peer loans and certain debt securities. HMRC publishes the qualifying-investments list in the ISA Managers Guidance Notes, not every retail product is eligible.
Transferring between providers
Use the receiving provider's ISA transfer form so tax-free status is preserved. Closing an account and re-depositing elsewhere counts as a new subscription and can breach the annual cap. Cash ISA transfers should complete within 15 business days; Stocks & Shares transfers often take longer. See the transfer guide for the full process.
Restrictions on partial transfers
Current-year subscriptions must transfer in full to the new provider. Prior-year balances may transfer partially or fully. Cross-type transfers (Cash to S&S, or into a Lifetime ISA) are permitted but Lifetime ISA transfers count against the £4,000 annual sub-limit.
Additional permitted subscriptions after a death
When a spouse or civil partner dies, the surviving partner may receive an additional ISA subscription allowance based on the deceased's ISA value. The rules differ by date of death, HMRC publishes the applicable form and deadline on gov.uk.
Death before 6 April 2018
The additional allowance equals the value of the deceased's ISAs at death, up to the normal annual subscription limit for that tax year. The surviving partner has three years from the date of death or 180 days after probate, whichever is later, to subscribe.
Death on or after 6 April 2018
The additional permitted subscription (APS) equals the total ISA value on the date of death and is not capped by the standard £20,000 limit. The surviving partner may subscribe to their own ISAs or those inherited from the deceased, subject to HMRC notification and the three-year / 180-day window.
Where to start
Guides
Editorial research
Cash ISA vs S&S, Long-run returns
Multi-decade analysis: cash 1.5-2% real vs S&S 4.5-5% real, volatility-adjusted.
LISA net-loss arithmetic
When the 25% bonus turns into net loss after early-withdrawal penalty, the 6.25% breakeven floor.
UK ISA wrapper mix shift 2014-2024
How the wrapper distribution shifted from 80% Cash to 60% Cash + 35% S&S over a decade.
Frequently Asked Questions
What does PlainISA do?
PlainISA gives you a free interactive UK ISA calculator and decision engine for the 2026/27 tax year. Enter your annual contribution, time horizon, ISA type, and expected return, the calculator projects your balance year by year, showing the impact of the £20,000 annual allowance, the 25% Lifetime ISA bonus (up to £1000/year), and compounding at your chosen return rate. The decision engine asks six short questions about your goal, time horizon, age, and risk tolerance, then recommends a Cash / S&S / Lifetime / Junior split with the reasoning shown.
What is the 2026/27 ISA allowance?
The total adult ISA subscription allowance for 2026/27 is £20,000. You can split this across Cash, Stocks & Shares, Lifetime, and Innovative-Finance ISAs in any combination, provided the Lifetime ISA element does not exceed £4,000. The Junior ISA allowance is separate: £9,000 per child, on top of the adult allowance, so a parent could subscribe £20,000 to their own ISA and £9,000 to each child's JISA in the same year. The allowance does not roll over; the unused portion at midnight on 5 April is gone.
What is the Lifetime ISA bonus and how does it actually work?
The government tops up your Lifetime ISA contributions by 25%. Contribute £4,000 in a tax year and HMRC adds £1,000, paid monthly into the account, not at year-end. The bonus is paid on contributions only (not on transfers in from old ISAs unless they count as new subscriptions). The catch is the 25% withdrawal charge: take money out for any reason other than a first-home purchase under £450,000, age 60+, or terminal illness, and HMRC claws back 25% of the entire balance, which is more than the original 25% bonus because the charge is applied to the topped-up amount.
Cash ISA or Stocks & Shares ISA, which is right for me?
It depends almost entirely on time horizon and your tolerance for short-term losses. Cash ISAs are the right choice for money you might need within 5 years and for any pot you cannot afford to see drop, the headline rate is guaranteed (currently around 4.5% for fixed-rate accounts) and FSCS-protected to £85,000 per institution. Stocks & Shares ISAs have averaged roughly 5% real returns over 30-year windows (FTSE All-Share Total Return Index, inflation-adjusted) but with substantial year-to-year volatility, they have lost 30% or more in single calendar years and recovered within 18-36 months. The decision engine quantifies this trade-off using your specific inputs; the calculator lets you stress-test different return assumptions.
Are ISA returns really tax-free?
Yes. Inside an ISA, interest, dividends, and capital gains are not taxable and do not need to be reported on a Self Assessment return. Outside an ISA, basic-rate taxpayers pay 8.75% on dividends above the £500 dividend allowance, higher-rate taxpayers 33.75%, and additional-rate taxpayers 39.35%. Capital gains above the £3,000 annual exempt amount are taxed at 18% (basic rate) or 24% (higher rate) on shares. For long-horizon investors in higher tax bands the ISA wrapper is materially valuable, a £20,000 contribution returning 5% real for 30 years compounds to roughly £86,000 inside an ISA versus roughly £68-72,000 in a taxable general investment account once dividend and CGT are deducted.
Can I have multiple ISAs of the same type?
Yes. Since 6 April 2024 you can open and contribute to multiple ISAs of the same type in the same tax year, for example, two Cash ISAs with different providers, provided your total contributions stay within the £20,000 overall allowance. The Lifetime ISA is the exception: you may still only subscribe to one Lifetime ISA per tax year, and you cannot hold more than one Lifetime ISA at a time (a transfer is required to switch providers).
Does PlainISA store anything I enter?
No. Every calculation runs in the browser; nothing is sent to a server. We have no account, no email field, no signup. Aggregate page-view metrics are collected via Umami analytics, no cookies, no personal identifiers, no third-party tracking pixels. The decision engine results are computed and rendered without leaving your device.
About this data
How PlainISA works, and why you can trust these numbers
What this site is
PlainISA is a free, independent UK ISA reference, calculator, and projection tool. It does not give personalised financial advice, and it does not recommend specific providers, funds, or platforms. PlainISA is an independent data-journalism publisher; we are not affiliated with HMRC, the Money and Pensions Service, MoneyHelper, or any ISA provider.
Editorial process
- Source. Pull the current ISA allowance, Lifetime ISA bonus rules, and account limits directly from gov.uk and the HMRC ISA Managers Guidance Notes each tax year.
- Verify. Cross-check historical return assumptions against the FTSE All-Share Total Return Index and protection limits against the Financial Services Compensation Scheme (FSCS), so every figure traces back to an official source.
- Publish. Build the calculator and decision engine to run entirely in the browser, nothing entered is sent to a server, and update the rules each time HMRC revises them.
Editorial independence & corrections
The PlainISA editorial process is independent and accepts no payment, sponsorship, or promoted placement from any ISA provider. Found an error or an out-of-date rule? File a correction via the contact page; we respond within 72 hours and publish corrections with a visible revision note. See our editorial standards and corrections policy for how we handle it, and our methodology for full source attribution and refresh cadence. PlainISA publishes ISA reference data for information only: it is not financial advice or a personal recommendation, projections are illustrations rather than forecasts, and tax rules and allowances change (appropriate use).
Frequently asked
What does PlainISA do?
PlainISA gives you a free interactive UK ISA calculator and decision engine for the 2026/27 tax year. Enter your annual contribution, time horizon, ISA type, and expected return, the calculator projects your balance year by year, showing the impact of the £20,000 annual allowance, the 25% Lifetime ISA bonus (up to £1000/year), and compounding at your chosen return rate. The decision engine asks six short questions about your goal, time horizon, age, and risk tolerance, then recommends a Cash / S&S / Lifetime / Junior split with the reasoning shown.
What is the 2026/27 ISA allowance?
The total adult ISA subscription allowance for 2026/27 is £20,000. You can split this across Cash, Stocks & Shares, Lifetime, and Innovative-Finance ISAs in any combination, provided the Lifetime ISA element does not exceed £4,000. The Junior ISA allowance is separate: £9,000 per child, on top of the adult allowance, so a parent could subscribe £20,000 to their own ISA and £9,000 to each child's JISA in the same year. The allowance does not roll over; the unused portion at midnight on 5 April is gone.
What is the Lifetime ISA bonus and how does it actually work?
The government tops up your Lifetime ISA contributions by 25%. Contribute £4,000 in a tax year and HMRC adds £1,000, paid monthly into the account, not at year-end. The bonus is paid on contributions only (not on transfers in from old ISAs unless they count as new subscriptions). The catch is the 25% withdrawal charge: take money out for any reason other than a first-home purchase under £450,000, age 60+, or terminal illness, and HMRC claws back 25% of the entire balance, which is more than the original 25% bonus because the charge is applied to the topped-up amount.
Cash ISA or Stocks & Shares ISA, which is right for me?
It depends almost entirely on time horizon and your tolerance for short-term losses. Cash ISAs are the right choice for money you might need within 5 years and for any pot you cannot afford to see drop, the headline rate is guaranteed (currently around 4.5% for fixed-rate accounts) and FSCS-protected to £85,000 per institution. Stocks & Shares ISAs have averaged roughly 5% real returns over 30-year windows (FTSE All-Share Total Return Index, inflation-adjusted) but with substantial year-to-year volatility, they have lost 30% or more in single calendar years and recovered within 18-36 months. The decision engine quantifies this trade-off using your specific inputs; the calculator lets you stress-test different return assumptions.
Are ISA returns really tax-free?
Yes. Inside an ISA, interest, dividends, and capital gains are not taxable and do not need to be reported on a Self Assessment return. Outside an ISA, basic-rate taxpayers pay 8.75% on dividends above the £500 dividend allowance, higher-rate taxpayers 33.75%, and additional-rate taxpayers 39.35%. Capital gains above the £3,000 annual exempt amount are taxed at 18% (basic rate) or 24% (higher rate) on shares. For long-horizon investors in higher tax bands the ISA wrapper is materially valuable, a £20,000 contribution returning 5% real for 30 years compounds to roughly £86,000 inside an ISA versus roughly £68-72,000 in a taxable general investment account once dividend and CGT are deducted.
Can I have multiple ISAs of the same type?
Yes. Since 6 April 2024 you can open and contribute to multiple ISAs of the same type in the same tax year, for example, two Cash ISAs with different providers, provided your total contributions stay within the £20,000 overall allowance. The Lifetime ISA is the exception: you may still only subscribe to one Lifetime ISA per tax year, and you cannot hold more than one Lifetime ISA at a time (a transfer is required to switch providers).
Does PlainISA store anything I enter?
No. Every calculation runs in the browser; nothing is sent to a server. We have no account, no email field, no signup. Aggregate page-view metrics are collected via Umami analytics, no cookies, no personal identifiers, no third-party tracking pixels. The decision engine results are computed and rendered without leaving your device.